Apple iPhone 3G

Yesterday made it the 5th time my screen went black after being idle....the phone is on b/c it rings....but no matter what I do the screen stays black....I have to put it on a charger several times or slide it off (which is fun since u cant see the screen) for it to stop...anyone else having this issue? And yes...the battery is fully charged, and no my phone has not been dropped or exposed to long periods of heat or anything of that nature.

Thanks!